The present invention relates generally to the problem of corrosion relating to buried and submerged metallic structures. In even greater particularity the present invention relates to an improved anti-corrosive protective material for use with cathodic protection systems. More particularly, the present invention relates to a protective material having microperforations that is used to encase buried conduits or other metallic structures to enhance cathodic protection and control microbiologically influenced corrosion.
Buried and submerged conduits, as well as other buried and submerged metallic structures such as tanks are ubiquitously used for storing and carrying various materials, such as water, natural gas, oil, and sewage. A major problem with buried and submerged metallic structures, or concrete with metal reinforcements, is corrosion. The severity and rate of corrosion is dependent on the type of material comprising the structure and the environment in which the structure is buried. Buried conduits in particular are an important part of the infrastructure in the United States and the world. Significant costs are involved in design, development, manufacture, and installation of water, gas, and sewage systems to ensure the longevity of buried conduits and other metallic structures. Failure of these systems from conduit corrosion represents appreciable costs. Corrosion of buried conduits and metallic structures is a major economic, environmental, and safety problem.
The control of corrosion of metals has been a quest of producers and consumers for the entire history of ferrous materials. Corrosion is the deterioration of a material, in most cases a metal, because of a reaction to its environment. Corrosion mainly occurs through an electrochemical process. For corrosion to occur, the following basic elements are required: an anode (the corroding metal); a cathode (the non-corroding metal); an electrically-conductive electrolyte in which the anode and cathode are immersed, such as ionized water or soil; and a current path connecting the anode and the cathode. Additionally, there must be an electric potential difference between the anode and cathode. When these elements are present, a complete electrical circuit referred to as a corrosion cell is formed. Through such a corrosion cell, an electric current flows from the anode to the cathode, carrying metallic ions with it and causing the anode to corrode. Corrosion protection systems seek to control corrosion by disrupting the operation of a corrosion cell.
Traditionally, buried and submerged metallic or metal reinforced structures are protected from corrosive environments by means of environmental barrier systems and/or cathodic protection systems. The materials used in barrier systems have high electrical resistance, and disrupt the corrosion cell by electrically isolating the metallic structure from the surrounding electrolyte environment. The types of barrier systems include 1) coatings that are tightly bound to the outer surface of the metallic structures, 2) wraps that are tightly fixed to the outer surface of the metallic structures using adhesives, and 3) loose polyethylene sleeves or wraps that are not bonded to the outer surface. Cathodic protection systems reduce corrosion by disrupting the difference in the electrical potential between the anode and the cathode. The principles of cathodic protection are based on minimizing the electrical potential between the anode and the cathode, as well as producing current flow in the proper direction to protect the metallic structure. This can be achieved by applying a current to the structure to be protected, such as a pipeline, from some outside source. When enough current is applied, the structure becomes a cathode in the corrosion cell, and thus a non-corroding metal. The types of cathodic protection systems are 1) impressed current systems whereby a current is supplied from an external power source or 2) sacrificial anode systems that rely on the corrosive potential of different metals, and include an anode that corrodes sacrificially to protect the metallic structure.
Bonded coatings on the outer surface of metallic structures are utilized extensively to provide barrier-type protection against corrosion. Despite their widespread use, there are significant disadvantages in the use of bonded coatings. One disadvantage is the tremendous expense associated with manufacturing of metallic structures having bonded coating. Placing a bonded coating on metallic structures requires several difficult and costly manufacturing steps, including the surface preparation of the metallic structures required prior to applying the coating, and the application process itself. These steps require the operation and maintenance of costly equipment used in metallic surface preparation and coating application. Further increasing the manufacturing costs of coated metallic structures are the costs of the coating materials, which can be significant. In addition to the initial expenses associated with manufacturing bonded coating structures, additional expenses are incurred by substantial and costly handling techniques for the finished products that are required to minimize damage to the coatings during the transportation and installation of coated metallic structures. Similar limitations are associated with wraps that are tightly fixed to the outer surface of the metallic structures using adhesives. Similar to bonded coatings, such wraps require significant costs associated with metallic surface preparation prior to application of the wraps, costs associated with the application process, and material costs associated with the use of adhesives.
Corrosion-control systems are often relied upon to protect very large surface areas of underground or submerged metallic structures, such as the case with cross-country pipelines. Another significant limitation of bonded coatings, in addition to the high costs associated with manufacturing and use, is that such coatings cannot be applied to large metallic surfaces with 100% coverage. Therefore, unprotected areas of pipe surfaces can occur, leading to disbanded areas of coating. Practice has demonstrated that even well-coated pipeline will have some coating defects, referred to as holidays, that expose bare metal to the corrosive effects of an electrolytic environment. Even if it were possible for a coated pipeline to be free of holidays, bare metal may be exposed by other physical means causing disbonding of the coating, such as pipe movements with temperature variations, soil stresses, and damage from outside sources during handling and burial. In contrast to a bonded coating system, loose polyethylene wrap, referred to as loose wraps, provide an inexpensive passive environmental barrier for corrosion control. Loose polyethylene wrap does not require metallic surface preparation, or the application and consumption of adhesive and coating materials.
To compensate for the coating defects associated with bonded coatings, it is common practice to use cathodic protection systems in conjunction with the use of bonded coating. The use of a barrier system such as bonded coatings with cathodic protection systems has the advantage of greatly reducing the operating costs of the cathodic protection system, because such a cathodic protection system need only protect the areas of bare metal exposed to the electrolytic environment at the holidays or other defects, rather than the whole surface of an uncoated metallic structure. Thus, the ongoing electrical energy consumption required to maintain cathodic protection for a coated structure can be significantly less than required to protect a bare, or uncoated structure. Cathodic protection systems can cause coatings and adhesives used with wraps to disbond from the metallic structures, causing the rate of electrical consumption utilized with a cathodic protection system to increase over time. It can be appreciated that the conventional use of bonded coatings and wraps in conjunction with cathodic protections systems comes with high costs, initially at the manufacturing-stage but continuing through increasing costs associated with the operation of the cathodic protection system.
Similar to the use of bonded coatings in conjunction with cathodic protection systems, it is advantageous to use loose wrap in conjunction with cathodic protection systems to reduce operating expenses and to provide enhanced corrosion protection. Traditionally, however, cathodic protection systems have not be used with loose wraps, because it is believed that loose wraps prevent the cathodic protection current from reaching the surface of the protected metallic structure. This belief has prevented the application of cathodic protection systems and technology with economically desirable loose wraps.
Another limitation in the prior art is the use of traditional bonded coatings or wraps with cathodic protection do not effectively address problems associated with microbiologically influenced corrosion (xe2x80x9cMICxe2x80x9d). Even after utilizing all the currently available mechanisms for the prevention of corrosion there are instances where unexplained corrosion occurs in buried metal pipes and conduits. In the 1930""s, the mechanisms of Microbiologically Influenced Corrosion (xe2x80x9cMICxe2x80x9d) were proposed by Von Wolzen Kuhr to explain corrosion initiated or accelerated by microorganisms. Since that time, studies have shown the Von Wolzen Kuhr theory to be partially valid, and it has been further established that a consortium of microorganisms contribute to many metal corrosion failures. These microorganisms, alone or more typically in combination, include as follows:
Sulphate-reducing bacteria including Desulfovibrio, Desulfobacter, and Desulformaculum. Sulphate-reducing bacteria are anaerobic and are the primary cause of Microbiologically Influenced Corrosion. Sulphate-reducing bacteria are associated with the reduction of sulphate under anaerobic conditions and an associated production of hydrogen sulfide, which creates an alkaline environment that can accelerate corrosion.
Iron-oxidizing bacteria including Gallionella, Sphaerotilus, Leptothrix, Clonothrix, and Crenotlxri. Iron-oxiding bacteria are associated with the oxidization of various forms of iron and, in some cases, an associated production of ferric chloride and an acidic environment that can accelerate corrosion.
Sulfur-oxidizing bacteria including Thiobacillus, Thiodendron, Beggiatoa, and Sulfolobus. Sulfur-oxiding bacteria are aerobic bacteria that form sulphuric acid, which is corrosive to many metals, from the oxidation of sulphur or sulphur-bearing compounds.
Slime-forming bacteria including Pseudomonas, Escherichia, Flavobacterium, Aerobacter and Bacillus.
Susan Watkins Borenstein, Microbiologically Influence Corrosion Handbook ch. 2 (1994).
Hereto, no one has effectively addressed the control of microbiologically influenced corrosion occurring in buried metal pipes and conduits, in conjunction with cathodic protection systems. For example, ductile iron pipe (DIP) typically exhibits a low risk to severe corrosion compared to other metals; however, a rapid increase in the corrosion rate can be initiated by oxygenated water, tidal action, or specific soil types such as soils containing sulfides. Because of the high costs associated with removal and replacement of corroded conduits, the industry has expended substantial resources attempting to solve this problem.
As discussed above, conduits have been traditionally covered with paint coatings, wraps, or other materials to separate the conduit surfaces from the environment. However, specialized coatings are either susceptible to deterioration by bacteria, including sulfate-reducing bacteria, or are sophisticated to the point that they are no longer cost effective. It is known that cathodic protection systems may be used to counteract microbiologically influenced corrosion, however, only by utilizing high current loads that require high ongoing energy costs. High current loads also have the disadvantage of accelerating the process of disbanding of coatings and adhesives, causing ever increasing energy consumption requirements to operate the cathodic protection system. The conventional use of cathodic protection systems in conjunction with bonded coatings or wraps does not provide effective and economical protection against microbiologically influenced corrosion.
Later, barrier films of polyethylene were used to protect DIP conduits. By insulating the exposed surfaces from soil, electrical currents, and oxygenated water, corrosion is usually controlled. However, due to improper installation, tears and punctures to the barrier film occurring during the installation and backfill process, free flow of water from tidal action, or soil or water becoming entrapped between the film and the conduit surface, actual corrosion still occurs in many cases. The industry has attempted to solve these problems by using more durable barrier films to encase the conduit surfaces, such as polyethylene and also others such as polypropylene, ethylene/vinyl acetate copolymers, vinyl acetate/vinyl chloride copolymers, and polyvinyl chloride.
The superior impact strength, tear resistance, and tensile strength of HDCLPE has reduced some of the problems associated with the installation and backfill process; however, all polyethylenes do not adequately address or control the problem of microbiologically influenced corrosion. Further, polyethylenes have a high electrical resistance and are not suitable without perforations for use with cathodic protection systems. Since heretofore there has not been an adequate alternative, present industry standards typically use either an 8 mil low density polyethylene (LDPE) film or a 4 mil HDCLPE film, a mil being equal to one thousandth of an inch (0.0254 millimeter), to wrap around the conduits for only barrier protection against corrosion.
Polyethylenes, as well as other plastic films, limit the free flow of water against the conduit surfaces, thereby reducing available oxygen. Any moisture that becomes trapped between the film and the conduit surface will eventually become deaerated. A problem arises where deaerated water levels are attained in the presence of the previously identified sulfate reducing bacteria. Many anaerobic bacteria, such as Desulfovibrio desulfuricans, thrive in certain fresh water, brackish water, sea water, sulfate soils, or warm soil conditions. These bacteria act as a catalyst to initiate or augment the rate of corrosion in an environment that is normally adverse to corrosion, and as previously noted are a primary cause of microbiologically influenced corrosion. Additionally, other types of bacteria are believed to play a part in corrosion propagation and it appears that bacteria can be responsible for degradation of some polyethylene film types. A possible solution to this problem is to treat the materials used to encase the conduit with bactericides. However, most bactericides are topical and water soluble, thereby offering only initial protection that loses their effectiveness when used in buried systems exposed to wet conditions. Since conduits are buried for decades, this would not provide adequate long-term protection.
Currently available methods do not allow the use of loose polyethylene wraps with cathodic protection systems. Further, current methods for controlling conduit corrosion do not address microbiologically influenced corrosion, or their application is technically complex and very expensive, or they are not suitable for buried conduits.
From the foregoing it may be seen that hereto, no one has adequately addressed the control of the microbiologically influenced corrosion element for the long-term protection of buried conduits. A need exists for an improved anti-corrosive material for protecting conduits buried in conditions favorable to microbiologically influenced corrosion.
It is an object of the present invention to provide an improved anti-corrosive material which is superior to those presently used to protect buried conduits.
A further object of the present invention is to provide a corrosion control system for buried and submerged metallic structures that is simple to install, resistant to installation damage, and requires no ongoing maintenance or monitoring except for any cathodic protection system that may be used therewith.
Another object of the present invention is to provide a loose polyethylene wrap that can effectively and economically control corrosion in the presence of cathodic protection systems.
A still further object of the present invention is to provide a corrosion control system that eliminates the need for bonded coatings on buried metallic structures, including pipelines and tanks.
Another object of the present invention is to provide a loose polyethylene encasement material have a plurality of microperforations.
An additional object of the present invention is to provide an improved anticorrosive material that provides long term protection of buried conduit by controlling microbiologically influenced corrosion.
Another object of the present invention is to provide a material that contains an antimicrobial additive to control bacterial-induced or enhanced corrosion.
A still further object of the present invention is to provide a corrosion control system that will provide protection against microbiologically influenced corrosion in the presence of cathodic protection systems and/or bond coated metallic structures having holidays.
Another object of the present invention is to provide a corrosion control system that provides for mitigation of corrosion resulting from installation damage.
An additional object of the present invention to provide an anti-corrosive material having an antimicrobial additive impregnated therein such that the antimicrobial additive can migrate within the material to contact the conduit surface and thereby prevent microbiologically influenced or enhanced corrosion.
These and other objects of the present invention are accomplished through the use of an improved anti-corrosive material used to protect buried conduits from corrosion. The improved anti-corrosive material is comprised of micro-perforated polyethylene encasement (MPPE). Such a loose polyethylene sleeve or wrap having a plurality of microperforations can be used in the presence of cathodic protection as would a bonded coating system. It can be used in lieu of expensive and easily-damaged bonded coatings on buried cathodically protected buried or submerged metallic structures, such as pipelines. Thus, the technology can be used to reduce the cathodic protection current requirements on bare (uncoated) or other metallic structures. The use of the present invention is more cost-effective than the use of bonded coatings because there are no special metal surface preparation requirements prior to use and no special handling requirements necessary to prevent damage to bonded coatings. Because no metal surface preparation is required, there is less likelihood of damage to metal surfaces during the manufacturing process. Further, the present invention does not require the use of adhesives or coatings, and can be applied either in the field or at a manufacturing facility.
Additionally, the present invention may be used with antimicrobial additives incorporated therein to effectively control bacteria associated with microbiologically influenced corrosion, without relying exclusively on cathodic protections systems for such protection. The additives can be blended with polyethylene raw materials in a manner that disperses the material throughout the product and be insoluble in aqueous conditions. Since the additive is designed as a contact agent, a requirement of manufacture is that proper dispersion and percent content be effected to generate the proper barrier against microbial growth. The anti-corrosive agents, or antimicrobial additives, are incorporated into and dispersed throughout the conduit contacting layer such that the anti-corrosive agents can migrate within the conduit contacting layer to contact the conduit surface and prevent corrosion. This product is a new cost effective anti-corrosive material used to protect buried conduits comprising ductile iron, steel, or concrete reinforced with steel by reducing or eliminating previously unexplained failures of buried conduits caused or accelerated by microbiologically influenced corrosion.
A test program was implemented to develop the proper polyethylene film manufacturing techniques that would yield proper protective characteristics in conjunction with the use of cathodic protection systems. This product is a new cost effective anti-corrosive material used to protect buried conduits comprising ductile iron, steel, or concrete reinforced with steel that may be used with cathodic protection systems.
The present invention provides an environmental barrier of loose polyethylene wrap that allows electrical current to flow through the wrap via a plurality of micro-perforations, or perforations, formed therein. The microperforations provide a mechanism to control microbiologically influenced corrosion in the presence of cathodic protection systems and/or disbonded coatings. The microperforations, having diameters from less than 1.0 rm to up to aproximately {fraction (1/8)} of an inch, are spaced along the wrap with intervals sufficient to allow adequate electrical pathways for appropriate cathodic protection system current flow. The present invention is not limited to simply overcoming the deficiencies of the prior art but introduces enhanced technical advantages in corrosion protection that were not previously available for the long term protection of buried conduits while continuing to meet established industry standards.
These and other objects and advantages of the invention will become apparent from the following detailed description of the preferred embodiment of the invention.